Light emitting diodes (LEDs) have been widely used in all aspects of industry and everyday life because of its small size, high efficiency and other characteristics.
FIG. 1 shows a common LED light emitting device. As show in FIG. 1, the LED light emitting device includes a light emitting circuit, a controllable switch Q1, a current detecting resistor Rsen, a thyristor U1 and a first current limiting resistor R1, wherein:
the light emitting circuit includes a plurality of LEDs D1, . . . Dn connected in series; where n is an integer greater than one;
an end of the thyristor U1 is connected to ground, and another end is connected to a driving end Vdrive providing a driving voltage via the first current limiting resistor R1;
a source electrode of the controllable switch Q1 is connected to the light emitting circuit, and a drain electrode of the controllable switch is connected to ground via the resistor Rsen, and a controlling end (gate electrode) of the controllable switch is connected to a first node X located between the first thyristor U1 and the first current limiting resistor R1.
However, there is a defect that protection for the driving circuit is not enough in the above LED light emitting device when an LED short circuit occurs. The description will be given below in connection with its work process as follows.
As shown in FIG. 1, the driving voltage Vdrive provides an ON voltage to turn on Q1, thereby a current flows through D1˜Dn and Rsen; a voltage drop will be generated when the current flows through Rsen; when a voltage drop across Rsen is greater than a threshold (e.g. 2.5 V), the thyristor U1 is turned on, thereby lowering the gate voltage of Q1, so that Q1 is turned off, that is, the LED current is turned off. After Q1 is turned off, the voltage drop across Rsen is 0, therefore the thyristor U1 is turned off again, making that Q1 is turned on again by the driving voltage Vdrive. Above process is repeated so that the voltage drop across Rsen can be maintained at 2.5 V.
However, there is another problem that an efficiency of the circuit is low in the circuit described above, which will be explained as follows.
Assumed that a total voltage drop of D1 . . . Dn is 30 V, Vdrive is 10 V, a reference voltage VRef of U1 is 2.5 V, the LED current is 250 mA, and then the resistor Rsen is 10 Ohms.
Generally, an operating current of U1 is 1˜100 mA. If the operating current of U1 is taken to be 50 mA, R1=Vdrive/50 mA=200 Ohms.
A total power of the circuit shown in FIG. 1 consists of the following components:
the power of R1: V2drive/R1=100/200=0.5 W;
the power of the light emitting circuit: 30V (the total voltage drop of D1 . . . Dn)*250 mA (the current of the light emitting circuit)=7.5 W;
the power of Rsen: 2.5 V (voltage drop of Rsen)*250 mA (the current of the light emitting circuit)=0.625 W.
Therefore, a circuit efficiency of the circuit shown in FIG. 1 is the power of the light emitting circuit/the total power of the circuit=7.5/8.625, approximately 87%.
The greater the current of the light emitting circuit is, the lower the circuit efficiency is.
Above is described with taking the LED light emitting circuit as an example, however, it should be understood that, there are same problems in similar circuits composed of other light emitting elements, which will not be described herein.